The subject of this invention is a method of receiving CDMA signals with parallel interference suppression, a corresponding stage and a corresponding receiver.
It finds application notable in radiocommunication with mobiles.
The technology of spectrum spreading by a direct sequence consists, schematically of multiplying an information symbol (for example a binary element) by a pseudo-random sequence (also called a code) made up of a sequence of elements called xe2x80x9cchipsxe2x80x9d. This operation has the effect of spreading the spectrum of the signal. On reception, the received signal is processed by correlation (or matched filtering) with a pseudo-random sequence identical to that of the transmission, which has the effect of reducing (or correlating) the spectrum. The signal correlated in this way, is processed in order to recover the information symbol.
This technique allows several users to access a single radiocommunications system, with the condition that they use distinct codes. One is then speaking of xe2x80x9cCode Division Multiple Accessxe2x80x9d or CDMA for short.
Despite offering numerous advantages, communications by spectrum spreading with code division multiple access are of limited capacity in terms of the number of users. This limitation is due to interference occurring between signals coming from different users. The more numerous the users are, the more important this interference phenomenon becomes.
Various solutions have been proposed to remedy this disadvantage and, notably, the suppression (or at the very least the reduction) of interference. Hence, in American patent U.S. Pat. No. 5,218,619, for example, sequential suppression of the interference is recommended proceeding by decreasing order of power of the signals from the various users. In American patent U.S. Pat. No. 5,363,403 contrary to this, parallel suppression of these interference signals is recommended. As this invention again takes up this latter technique, we can break off there and illustrate the general structure of a receiver of this type.
The receiver illustrated in the appended FIG. 1 comprises a general input E receiving a composite signal r(t) formed from a plurality of signals corresponding to different information symbols S1, S2, S3 which have been spread by a plurality of pseudo-random codes C1, C2, C3. The receiver shown is assumed to work with three codes but in practice, obviously, this number is higher.
The receiver firstly comprises an input stage with means 101, 102, 103 capable of receiving the composite signal and of supplying a signal correlated by the code C1, C2 or C3 appropriate to each channel; these means can consist of a correlator or a matched filter.
Next the receiver comprises a parallel interference suppression stage 100 which comprises :
means 111, 112, 113 of receiving the correlated signal and supplying an estimation Ŝ1, Ŝ2, or Ŝ3 of the corresponding information symbol; these means can comprise an integrator and a decision circuit
means 121, 122, 123 capable of respreading the estimated symbol Ŝ1, Ŝ2, or Ŝ3 using the code C1, C2, C3 appropriate to the channel, to supply the respread signals s1, s2 or s3.
means 131, 132, 133 to subtract from the signal applied to the input of the channel (after a suitable delay produced by a delay circuit 161, 162, 163), the sum xcexa31, xcexa32, xcexa33 of the respread signals coming from the other channels; in other words, the signal xcexa31 is formed by the sum s2+s3, the signal xcexa32 by s1+s3 and the signal xcexa33 by s1+s2. The means 131, 132, 133 supply, in each channel, a new signal r1, r2, r3 which, at least in part, has been cleared of multiple access interference corresponding to other channels.
After the parallel interference suppression stage, there are three matched filters 201, 202, 203 working respectively with the codes C1, C2, C3 and correlating the signals r1, r2, r3 then an output stage 200 with three decision circuits 211, 212, 213 supplying the three data S1, S2 and S3.
Although giving satisfaction in certain regards, such receivers do not eliminate the risks of error. The suppression of interference, if it is carried out without precautions, can even increase this risk. The purpose of this invention is precisely to reduce this risk (in other words to reduce the bit error rate), by improving the means of reconstructing the signals before the actual interference suppression itself. With the invention, a single parallel interference suppression stage offers better performance than the traditional two suppression stages.
In order to obtain this result in the interference suppression stage and to estimate the received data, the invention provides for the use of a particular criterion which is called xe2x80x9cThe Maximum Likelihoodxe2x80x9d criterion. This criterion is known of itself in CDMA techniques. One may find a description for example in the work by J. G. PROAKIS entitled xe2x80x9cDigital Communicationsxe2x80x9d McGRAW-HILL Inc., 3rd edition, 1995, Chapter 5-1-4. However, in the prior art, this criterion is used in an ordinary receiver, and not in a means of parallel suppression of multiple access interference. Furthermore, in the prior art, this criterion is used with the aid of an algorithm called Viterbi""s Algorithm, which allows one to find, through a lattice representing all possible configurations, a sequence of data which minimizes a quantity called the xe2x80x9cEuclidean distance metricxe2x80x9d. This technique, which takes into account the whole of the data transmitted by all users, is often very complex. This invention adapts this technique notably by simplifying it. Furthermore it defines a metric which is particularly suitable for the parallel suppression of multiple access interference.
Put precisely, the subject of this invention is a method of receiving CDMA signals with parallel interference suppression in which:
a composite signal is received comprising a plurality of K signals corresponding to information symbols which have been spread in frequency by K different pseudo-random sequences,
these K signals are correlated using said K sequences
the corresponding K symbols are estimated,
the K correlated signals are reconstructed in frequency by respreading said estimated symbols using the corresponding pseudo-random sequences,
the contributions of the other signals are subtracted from a respread signal to provide K new signals, spread in frequency but cleared, at least in part of the interference,
this method being characterized in that:
all the possible hypotheses possible are formulated on the signs of the NK correlated signals, where N is a whole number equal to 1 or to a few units,
for each hypothesis, one calculates the distance metric between the group of correlated signals undergoing processing and the corresponding signals before processing,
the hypothesis for which the metric is the smallest is retained, being the hypothesis which has a maximum likelihood,
only those signals corresponding to this maximum likelihood hypothesis are reconstructed.
Another subject of the invention is a parallel interference suppression stage that implements this method, this stage comprising:
K inputs receiving signals correlated in frequency,
K means of estimating K symbols corresponding to these K signals,
K means of reconstructing signals respread in frequency using the corresponding pseudo-random sequences,
means of parallel interference suppression comprising K channels in parallel capable of subtracting from one respread signal, the contributions of the other respread signals,
K outputs supplying K signals spread in frequency, cleared, at least in part of the interference,
this stage being characterized in that it comprises
means placed between the estimation means and the reconstruction means and capable of formulating all the possible hypotheses on the signs of NK correlated signals, where N is a whole number equal to 1 or to a few units, and of calculating, for each hypothesis, the distance metric between the group of correlated signals undergoing processing and the corresponding signals before processing, and of retaining the hypothesis for which the metric is the smallest, the hypothesis which offers a maximum likelihood.
Another subject of the invention is a receiver for CDMA signals that implements the method defined above and comprising:
a general input capable of receiving a composite signal formed from a plurality of K signals corresponding to information symbols that have been spread in frequency by K different pseudo-random sequences,
an input stage with K channels in parallel each comprising filters to correlate in frequency the composite signal through one of the K pseudo-random sequences, this stage supplying K signals correlated in frequency,
at least one parallel interference suppression stage,
filter stages positioned between the parallel interference suppression stages and comprising K filters matched to the pseudo-random sequences,
an output circuit comprising K decision circuits,
this receiver being characterized in that at least one of the parallel interference suppression stages is a stage such as that defined above.